As the above humidity indicator, there is a conventionally known structure in which a humidity determining plate, formed by impregnating a card-shaped base paper with a moisture-sensitive material that changes color due to absorption of moisture (e.g. cobalt chloride), has provided on the front face a determining face as a humidity determining section where the moisture-sensitive material is exposed, and determination of the humidity in the environment around the base paper is carried out visually by a change in color (in the case of cobalt chloride, blue to pink) of the moisture-sensitive material on the determining face.
Such a humidity indicator is used in the transport, etc. of various types of industrial products that dislike moisture (e.g. electronic components such as circuit boards in which cracks may be caused by an epoxy-based resin absorbing moisture) by being enclosed together with the product and a desiccant pack within a transparent gas-tight packaging bag. That is, when the product is transported in such a usage state, since whether or not the humidity within the packaging bag exceeds a specified limit can be determined visually from the color of the humidity determining face of the humidity indicator, it is possible to simply check whether or not the interior of the packaging bag containing the desiccant pack has been kept in appropriate humidity conditions (dry conditions), and because of this such a humidity indicator is conventionally used widely in the electronic industry, etc.